Studies with animals have shown that stimuli associated with drug delivery can come to function as variables that partially control drug-seeking behavior and the likelihood of resumption (i.e., relapse) to such behavior, even in the absence of the drug. Analogous research strategies are being used to assess the generality of these findings to human subjects. In addition, these procedures provide data on the degree of correspondence between self-reported drug effects and drug seeking behavior. The human studies have produced a number of interesting results. When the consequences of varying the dose of morphine available on self-administration, physiological effects, and self-reported effects were examined, it was found that low doses of morphine (3.75 mg) maintained rates of responding above placebo and constricted pupillary diameter, but did not reliably alter the self-reports of the subjects, indicating a dissociation between the subjective effects of morphine and morphine's reinforcing properties. Another study evaluated the role of a stimuli paired with drug administration on the maintenance of responding. Initial results suggested that the stimuli were of less importance than in an analogous study with animals, as well as in somewhat similar study of cocaine self-administration by humans.